When a digital signal flow composed of a plurality of time-division multiplexed channels (TDM=Time Division Multiplex) arrives at a receiving terminal, it must undergo a demultiplexing operation allowing the extraction of bits belonging to a determined channel. If the transmission is carried out by sending on an optical fiber a light radiation amplitude-modulated by the digital TDM flow, the demultiplexing at the receiving terminal can be carried out according to two different methods:
by converting the whole TDM data flow from optical signals into electrical signals and demultiplexing the signal obtained by suitable electronic circuits of the type commonly used in digital telecommunication apparatus; and PA1 by extracting from the TDM data flow only the channel desired by optical devices and converting it into an electric signal for the successive processing operations. PA1 --an electro-optical switch, of the type described by R. S. Tucker et al in "16 Gbit/s fiber transmission experiment using optical time-division multiplexing", Electronic Letters, Vol. 23, n.24, pages 1270-1271; PA1 --a semiconductor optical amplifier, switched on only in correspondence with the tributary to be extracted, according to the method described by P. B. Hansen et al in "Optical Demultiplexing at 6 Gbit/s Using a Semiconductor Laser Amplifier as an Optical Gate" IEEE Photonics Technology Letters, Vol. 3, NO. 11, November 1991, pages 1018-1020; PA1 --an optical fiber trunk, whereto the TDM data flow and a suitable sampling signal are sent at the same time and wherefrom the relevant conversion products, generated by non-linearity phenomena, are extracted by a suitable optical filtering operation. This method is described by P. A. Andrekson et al in "All-Optical Demultiplexing at 16 Gbit/s using Four-Wave Mixing", issued in Technical Digest of OFC'91, San Diego 1991, pages 19-22. PA1 --an optical coupler to which the optical carrier at a first wavelength and one or more sampling optical carriers at different wavelengths are sent, consisting of carriers reaching their maximum amplitude only in the time intervals which the relevant tributary to be extracted is present; PA1 --one or more sources generating the sampling optical carriers; PA1 --a semiconductor optical amplifier, whose input receives the sum of the optical carriers present at the output of said optical coupler, the amplifier gain decreasing with the increasing of the total power of input carriers; PA1 --a second optical coupler which subdivides the power supplied at the output by the optical amplifier; PA1 --one or more optical filters with pass band centered on the wavelength of one of the sampling optical carriers to whose input there is sent the signal supplied by the second optical coupler and whose output supplies the tributary to the receivers.
The first method, which is presently the most widely used, requires a broadband optical receiver and demultiplexing electronic circuits operating at the bit rate of the TDM data flow. Good performances in terms of sensitivity are difficult to achieve by the use of wide-band optical receivers and the electronic circuits are particularly critical, especially when operating at bit rates of some Gbit/s.
The second method demands a suitable device which allows the optical signal to be deviated to the receiver exclusively at the instants corresponding to the passage of the bits belonging to the channel to be extracted. Such a device must be capable of operating at a rate comparable with TDM data flow bit rate, requiring as low as possible optical power levels.
Different methods using devices for this purpose have been described in the literature. Such devices include:
The first two methods dramatically degrade TDM data flow integrity, rendering it non-available for further processing operations, while the third requires high signal powers, of the order of 100 mW, and optical signals with wavelengths very close to one another, so as to obtain the desired non-linearity effects in the fiber. The latter requirement demands also the use of very narrow optical filters and highly wavelength-stabilized optical-carriers.